1. Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to the transmission and reception of ACKnowledgements (ACK) signals.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from transmission points, such as, for example, Base Stations (BSs), or NodeBs, to User Equipments (UEs). The communication system also includes an UpLink (UL) that conveys transmission signals from UEs to reception points, such as, for example BSs or NodeBs. A UE, which is also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be embodied as a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as an access point or some other equivalent terminology.
DL signals consist of data signals carrying information content, control signals carrying DL Control Information (DCI), and Reference Signals (RSs), which are also known as pilot signals. A NodeB transmits data information or DCI to UEs through a Physical DL Shared CHannel (PDSCH) or a Physical DL Control CHannel (PDCCH), respectively.
UL signals also consist of data signals, control signals and RSs. A UE transmits data information or UL Control Information (UCI) to a NodeB through a Physical Uplink Shared CHannel (PUSCH) or a Physical Uplink Control CHannel (PUCCH), respectively.
A NodeB transmits one or more of multiple types of RSs, including a UE-Common RS (CRS), a Channel State Information RS (CSI-RS), and a DeModulation RS (DMRS). The CRS is transmitted over substantially the entire DL system BandWidth (BW), and can be used by all UEs to demodulate data or control signals or to perform measurements. A UE can determine a number of NodeB antenna ports from which a CRS is transmitted through a broadcast channel transmitted from the NodeB. To reduce the overhead associated with the CRS, a NodeB may transmit a CSI-RS with a density in the time and/or frequency domain that is smaller than that of the CRS, for UEs to perform measurements. A UE can determine the CSI-RS transmission parameters through higher layer signaling from the NodeB. DMRS is transmitted only in the BW of a respective PDSCH, and a UE can use the DMRS to demodulate the information in the PDSCH.
A PDSCH transmission to a UE, or a PUSCH transmission from a UE, may be in response to dynamic scheduling or Semi-Persistent Scheduling (SPS). In dynamic scheduling, a NodeB conveys, to a UE, a DCI format through a respective PDCCH. The contents of a DCI format, and consequently its size, depend on the Transmission Mode (TM) for which a UE is configured for a respective PDSCH reception or PUSCH transmission. In SPS, a PDSCH or a PUSCH transmission is configured to a UE by a NodeB through higher layer signaling, such as, for example, Radio Resource Control (RRC) signaling. The transmission occurs at predetermined time instances and with predetermined parameters, as informed by the higher layer signaling.
FIG. 1 is a diagram illustrating a structure for a DL Transmission Time Interval (TTI).
Referring to FIG. 1, a DL TTI includes one subframe 110, which includes two slots 120 and a total of NsymbDL symbols for transmitting data information, DCI, or RS. Orthogonal Frequency Division Multiplexing (OFDM) is assumed for DL signal transmissions, and an OFDM symbol includes a Cyclic Prefix (CP). A first MsymbDL symbols are used to transmit DL CCHs 130. These MsymbDL symbols may be dynamically indicated in each DL TTI through a Physical Control Format Indicator CHannel (PCFICH) transmitted in a first subframe symbol. Remaining MsymbDL-MsymbDL symbols are mainly used to transmit PDSCHs 140. A transmission BW consists of frequency resource units referred to as Resource Blocks (RBs). Each RB includes NscRB sub-carriers, or Resource Elements (REs). A unit of one RB in the frequency domain and one subframe in the time domain is referred to as a Physical Resource Block (PRB). A UE is allocated MPDSCH m RBs for a total of MscPDSCH=MPDSCH·NscRB REs for a PDSCH transmission BW. Some REs in some symbols contain CRS 150 (or DMRS), which enable channel estimation and coherent demodulation of information signals at a UE.
Additional control channels may be transmitted in a DL control region. For example, assuming use of a Hybrid Automatic Repeat reQuest (HARQ) process for data transmission in a PUSCH, a NodeB may transmit HARQ-ACK information in a Physical Hybrid-HARQ Indicator CHannel (PHICH) to indicate to a UE whether its previous transmission of each data Transport Block (TB) in a PUSCH was correctly detected (i.e. through an ACK) or incorrectly detected (i.e. through a Negative ACK (NACK)).
FIG. 2 is a diagram illustrating an encoding process for a DCI format at a NodeB transmitter.
Referring to FIG. 2, a NodeB separately codes and transmits each DCI format in a respective PDCCH. A Cell or SPS Radio Network Temporary Identifier (C-RNTI or SPS-RNTI) for a UE, for which a DCI format is intended for, masks a Cyclic Redundancy Check (CRC) of a DCI format codeword in order to enable the UE to identify that a particular DCI format is intended for the UE. Alternatively, a DCI-type RNTI may mask a CRC if a DCI format provides UE-common information. A CRC computation unit 220 computes the CRC of (non-coded) DCI format bits 210. The CRC is then masked using an exclusive OR (XOR) operation 230 between the CRC and respective RNTI bits 240. The XOR operation 230 is defined as: XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. For example, both a CRC and an RNTI consist of 16 bits. The masked CRC bits are appended to DCI format information bits using a CRC append operation at an append CRC unit 250. Channel coding is performed using a channel coding operation at a channel coding unit 260 (for example, using a convolutional code). A rate matching operation is performed to allocated resources at a rate matching unit 270. Interleaving and modulation are performed at an interleaving and modulation unit 280 for transmission of a control signal 290.
FIG. 3 is a diagram illustrating a decoding process for a DCI format at a UE receiver.
Referring to FIG. 3, a UE receiver demodulates a received control signal 310 and resulting bits are de-interleaved at a demodulation and de-interleaving unit 320. A rate matching applied at a NodeB transmitter is restored through a rate matching unit 330. Data is subsequently decoded at a channel decoder 340. After decoding the data, DCI format information bits 360 are obtained after extracting CRC bits at a CRC extraction unit 350. The CRC bits are de-masked by applying an XOR operation 370 with a respective UE RNTI mask 380. A UE performs a CRC test in a CRC test unit 390. If the CRC test passes, a UE considers the DCI format as valid and determines parameters for signal reception or signal transmission. If the CRC test does not pass, a UE disregards the presumed DCI format.
To avoid a PDCCH transmission to a UE that is blocking a PDCCH transmission to another UE, a location of each PDCCH in the time-frequency domain of a DL control region is not unique. Therefore, a UE needs to perform multiple decoding operations to determine whether there are PDCCHs intended for the UE in a DL subframe. The REs carrying a PDCCH are grouped into Control Channel Elements (CCEs) in the logical domain. For a given number of DCI format bits in FIG. 2, a number of CCEs for a respective PDCCH depends on a channel coding rate (Quadrature Phase Shift Keying (QPSK) is assumed as the modulation scheme). A NodeB may use a lower channel coding rate (i.e., more CCEs) for transmitting PDCCHs to UEs experiencing a low DL Signal-to-Interference and Noise Ratio (SINR) than to UEs experiencing a high DL SINR. The CCE aggregation levels may include, for example, of LC∈{1,2,4,8} CCEs.
For a PDCCH decoding process, a UE may determine a search space for candidate PDCCHs after the UE restores the CCEs in the logical domain, according to a common set of CCEs for all UEs (i.e., a Common Search Space (CSS)) and according to a UE-dedicated set of CCEs (i.e., a UE-Dedicated Search Space (UE-DSS)). A CSS may include the first C CCEs in the logical domain. A UE-DSS may be determined according to a pseudo-random function having UE-common parameters as inputs, such as, for example, the subframe number or the total number of CCEs in the subframe, and UE-specific parameters such as the RNTI. For example, for CCE aggregation levels LC∈{1,2,4,8}, the CCEs corresponding to PDCCH candidate m are provided by Equation (1).CCEs for PDCCH candidate m=L·{(Yk+m) mod └NCCE,k/L┘}+i  (1)In Equation (1), NCCE,k is a total number of CCEs in subframe k, i=0, . . . , LC−1, m=0, . . . , MC(LC)−1, and MC(LC) is a number of PDCCH candidates to monitor in a search space. For example, for LC∈{1,2,4,8}, MC(LC)={6,6,2,2}, respectively. For the CSS, Yk=0. For the UE-DSS, Yk=(A·Yk-1) mod D where Y−1=RNTI≠0, A=39827 and D=65537.
DCI formats conveying information to multiple UEs are transmitted in a CSS. Additionally, if enough CCEs remain after the transmission of DCI formats conveying information to multiple UEs, a CSS may also convey some UE-specific DCI formats for DL SAs or UL SAs. A UE-DSS exclusively conveys UE-specific DCI formats for DL SAs or UL SAs. For example, a UE-CSS may include 16 CCEs and support 2 DCI formats with L=8 CCEs, 4 DCI formats with L=4 CCEs, or 1 DCI format with L=8 CCEs and 2 DCI formats with L=4 CCEs. The CCEs for a CSS are placed first in the logical domain (prior to interleaving).
FIG. 4 is a diagram illustrating a transmission process of DCI formats in respective PDCCHs.
Referring to FIG. 4, encoded DCI format bits are mapped to PDCCH CCEs in the logical domain. The first 4 CCEs (L=4), CCE1 401, CCE2 402, CCE3 403, and CCE4 404 are used to transmit a PDCCH to UE1. The next 2 CCEs (L=2), CCE5 411 and CCE6 412, are used to transmit a PDCCH to UE2. The next 2 CCEs (L=2), CCE7 421 and CCE8 422, are used to transmit a PDCCH to UE3. Finally, the last CCE (L=1), CCE9 431, is used to transmit a PDCCH to UE4. The DCI format bits may be scrambled by a binary scrambling code, in step 440, and are subsequently modulated, in step 450. Each CCE is further divided into Resource Element Groups (REGs). For example, a CCE consisting of 36 REs can be divided into 9 REGs, each consisting of 4 REs. Interleaving is applied among REGs (blocks of 4 QPSK symbols), in step 460. For example, a block interleaver may be used. The resulting series of QPSK symbols may be shifted by J symbols, in step 470. Each QPSK symbol is mapped to an RE in the control region of the DL subframe, in step 480. Therefore, in addition to a CRS, 491 and 492, and other control channels such as a PCFICH 493 and the PHICH, the REs in the PDCCH contain QPSK symbols corresponding to a DCI format for UE1 494, UE2 495, UE3 496, and UE4 497.
A UE may transmit a HARQ-ACK signal in a PUCCH in response to detecting a PDCCH associated with a PDSCH, and may implicitly derive a respective PUCCH resource nPUCCH from the first CCE, nCCE of a respective PDCCH as set forth in Equation (2).nPUCCH=nCCE+NPUCCH  (2)where NPUCCH is an offset the NodeB informed to UEs through higher layer signaling.
For a UL system BW consisting of NRBmax,UL RBs, where each RB consists of NscRB=12 REs, a Zadoff-Chu (ZC) sequence ru,v(α)(n) can be defined by a Cyclic Shift (CS) α of a base ZC sequence ru,v(n) according to ru,v(α)(n)=ejanru,v(n), 0≤n<MscRS, where MscRS=mNscRB is the length of the ZC sequence, 1≤m≤NRBmax,UL, and ru,v(n)=xq (n mod NZCRS) where the qth root ZC sequence is defined by
                    x        q            ⁡              (        m        )              =          exp      ⁡              (                                            -              j                        ⁢                                                  ⁢            π            ⁢                                                  ⁢                          qm              ⁡                              (                                  m                  +                  1                                )                                                          N            ZC            RS                          )              ,0≤m≤NZCRS−1 with q given by q=└q+½┘+v·(−1)└2q┘ and q given by q=NZCRS·(u+1)/31. A length NZCRS of a ZC sequence is given by the largest prime number such that NZCRS<MscRS. Multiple RS sequences can be defined from a single base sequence through different values of α. A PUCCH transmission is assumed to be in one RB (MscRS=NscRB).
FIG. 5 is a block diagram illustrating a UE transmitter for a ZC sequence.
Referring to FIG. 5, a sub-carrier mapping unit 520 maps a ZC sequence from a ZC sequence unit 510 to REs of an assigned transmission BW as they are indicated by RE selection unit 525. Subsequently, an IFFT is performed by an IFFT unit 530, a CS is applied to the output by a CS unit 540, followed by scrambling with a cell-specific sequence using a scrambling unit 550. A CP is inserted by a CP insertion unit 560, and the resulting signal is filtered by a time windowing unit 570. The transmission power PPUCCH is applied by a power amplifier 580, and a ZC sequence 590 is transmitted. Without modulation, a ZC sequence serves as an RS. With modulation, a ZC sequence serves as a HARQ-ACK signal.
The DL control region in FIG. 1 uses a maximum of MsymbDL=3 OFDM symbols and transmits a control signal substantially over a total DL BW. This configuration limits PDCCH capacity and cannot achieve interference coordination in the frequency domain among PDCCH transmissions from different NodeBs. There are several cases where expanded PDCCH capacity or PDCCH interference coordination in the frequency domain is needed for transmission of control signals. One such case is use of spatial multiplexing for PDSCH transmissions where multiple PDCCHs schedule same PDSCH resources to multiple UEs and expanded PDCCH capacity is needed. Another case is for heterogeneous networks where DL transmissions in a first cell experience strong interference from DL transmissions in a second cell, and interference coordination in the frequency domain between the two cells is needed.
A direct extension of the maximum DL control region size to more than MsymbDL=3 OFDM symbols is not possible at least due to the requirement to support legacy UEs, which cannot be aware of such an extension. An alternative is to support DL control signaling in the conventional PDSCH region by using individual PRBs. A PDCCH transmitted in PRBs of the conventional PDSCH region are referred to as Enhanced PDCCH (EPDCCH).
FIG. 6 is a diagram illustrating EPDCCH transmissions in a DL subframe.
Referring to FIG. 6, although EPDCCH transmissions start immediately after a conventional DL control channel 610 and are transmitted over all remaining DL subframe symbols, EPDCCH transmissions may instead start at a predetermined subframe symbol and extend over a part of remaining DL subframe symbols. EPDCCH transmissions may occur in four PRBs, 620, 630, 640, and 650, while remaining PRBs 660, 662, 664, 666, and 668 may be used for PDSCH transmissions. As an EPDCCH transmission over a given number of subframe symbols may require fewer REs than the number of subframe symbols available in a PRB, multiple EPDCCHs may be multiplexed in a same PRB. The multiplexing can be in any combination of possible domains (i.e., time domain, frequency domain, or spatial domain) and, in a manner similar to a PDCCH, an EPDCCH includes at least one Enhanced CCE (ECCE). Similar extensions may apply to PCFICH transmission (EPCFICH) or to PHICH transmission (EPHICH).
A UE can be configured by higher layer signaling the PRBs for potential transmissions of Enhanced CCHs (ECCHs), which can include, for example, EPDCCHs, EPCFICH, or EPHICHs. An ECCH transmission to a UE over a number of DL subframe symbols may be in a single PRB, if a NodeB has accurate DL channel information for the UE and can perform Frequency Domain Scheduling (FDS) or beam-forming, or it may be in multiple PRBs if accurate DL channel information is not available or if an ECCH is intended for multiple UEs. An ECCH transmission over a single PRB is referred to as localized or non-interleaved. An ECCH transmission over multiple PRBs is referred to as distributed or interleaved.
An exact design of a search space for EPDCCH candidates is not material to embodiments of the present invention and may be assumed to follow the same principles as a search space design for PDCCH candidates. Therefore, a number of EPDCCH candidates can exist for each possible ECCE aggregation level LE where, for example, LE∈{1,2,4} ECCEs for localized EPDCCH and LE∈{1,2,4,8} ECCEs for distributed EPDCCH. A UE determines EPDCCH candidates for each ECCE aggregation level in a search space according to predetermined functions similar to the one previously described for determining CPDCCH candidates for each CCE aggregation level.
FIG. 7 is a diagram illustrating an allocation of ECCEs for localized EPDCCH transmissions.
Referring to FIG. 7, the partitioning of ECCEs is in the frequency domain, a FRB contains 4 ECCEs, 710, 720, 730, and 740, and an EPDCCH transmission to a UE may consist of 1, 2, or 4 ECCEs. There are four orthogonal DMRS antenna ports using Code Division Multiplexing (CDM) and Frequency Division Multiplexing (FDM). DMRS port 1 750 and DMRS port 2 760 occupy the same REs and are separate through the use of the Orthogonal Covering Codes (OCCs) {1, 1} and {1, −1}, respectively, over two successive subframe symbols. The same applies for DMRS port 3 770 and DMRS port 4 780, which occupy different REs than the first two DMRS ports. A DMRS transmission from each antenna port may also be scrambled with a scrambling sequence. For localized EPDCCH, a UE is assigned a unique DMRS port, based for example on its identity (C-RNTI) or the subframe number, or the DMRS antenna port for a UE may also depend on the ECCE number or the EPDCCH candidate. An EPDCCH transmission is assumed to start, for example, in a first subframe symbol after legacy CCHs 790, if any, and extend in the remaining subframe symbols.
To improve the spectral efficiency of EPDCCH transmissions and therefore reduce the associated overhead and increase the DL throughout, EPDCCHs to different UEs may be transmitted using spatial multiplexing. This is enabled by the NodeB opportunistically using the same resources for multiple EPDCCH transmissions to respectively multiple UEs by applying a different precoding to each EPDCCH transmission so that it becomes substantially orthogonal to the remaining EPDCCH transmissions, thereby substantially suppressing the mutual interference. In enabling spatial multiplexing, it is essential to provide orthogonal DMRS to each UE so that a respective channel estimate can be accurately obtained and orthogonal projections to the remaining EPDCCH transmissions can be made. In this manner, and as the DMRS conveyed by each EPDCCH has the same precoding as the respective data, the use of spatial multiplexing is transparent to a UE.
FIG. 8 is a diagram illustrating a transmission of two EPDCCHs through spatial multiplexing using same ECCEs.
Referring to FIG. 8, a first EPDCCH transmission associated with DMRS port 1 810 and a second EPDCCH transmission associated with DMRS port 2 820 are multiplexed in REs corresponding to the same ECCEs #0 and #1 830. DMRS port 3 860 and DMRS port 4 870 may or may not exist. In the latter case, the respective REs may be used for EPDCCH transmission (or may remain empty). Also, although the spatially multiplexed EPDCCH transmissions are shown to be transmitted over the same number of ECCEs, they may instead be transmitted over a different number of ECCEs and partially overlap. Similar to the DMRS, the control information in a DCI format can be scrambled by a scrambling sequence.
The use of spatial multiplexing for transmissions of EPDCCHs associated with PDSCHs to respective UEs results in PUCCH resource collision for respective HARQ-ACK signal transmissions under the conventional PUCCH resource determination. Denoting the first EPDCCH ECCE as nECCE the PUCCH resource for HARQ-ACK signal transmission is nPUCCHE=nECCE+NPUCCHE, where NPUCCHE is an offset a NodeB informed to UEs through higher layer signaling. NPUCCHE may be the same as NPUCCH or it may be separately configured for EPDCCH operation. When nECCE is the same for UEs with spatially multiplexed EPDCCH transmissions associated with respective PDSCHs, the PUCCH resource for each respective HARQ-ACK signal transmission is the same.
The previous PUCCH resource collision problem is further exacerbated when a UE is configured antenna transmission diversity for HARQ-ACK signal transmissions and a different PUCCH resource is required for each antenna. For two antennas, a conventional method is to obtain a PUCCH resource for the first antenna as for the case of a single antenna, nPUCCH=nECCE+NPUCCH, and obtain a PUCCH resource for the second antenna as nPUCCH=nECCE+1+NPUCCH. Due to the limited number of ECCEs per PRB, such as 4 ECCEs per PRB, the PUCCH resource collision problem for transmitter antenna diversity exists regardless of the use of spatial multiplexing for EPDCCH transmissions.
Regardless of whether spatial multiplexing is used for EPDCCH transmissions or transmitter antenna diversity is used for HARQ-ACK signal transmissions in response to an EPDCCH detection associated with a PDSCH, the channelization of respective PUCCH resources needs to be defined. These PUCCH resources in response to detections of EPDCCHs and in response to detections of PDCCHs can be shared or separate. Moreover, these PUCCH resources in response to detections of distributed EPDCCHs and in response to detections of localized EPDCCHs can be also shared or separate. In general, separate PUCCH resources increase UL overhead since the number of PDSCHs per subframe does not significantly vary regardless of whether the scheduling is only by PDCCHs, only by EPDCCHs, or by both.
In case a PUCCH resource nPUCCH in response to an EPDCCH detection associated with a PDSCH, is implicitly derived as a function of the first ECCE nECCE and a NPUCCHE parameter configured by higher layer signaling, nPUCCHE=f(nECCE)=nECCE+NPUCCHE, collisions among PUCCH resources used in response to PDCCH and EPDCCH detections by different UEs can be avoided by either one of the following approaches:    a) The values of NPUCCH and NPUCCHE are such that different PUCCH resources are always used for HARQ-ACK signal transmissions corresponding to PDCCH and EPDCCH detections, respectively.    b) A UE decodes a PCFICH and determines a total number of PDCCH CCEs (by determining a number of DL subframe symbols used to transmit legacy DL control region and knowing a number of CRS REs and PHICH/PCFICH REs). PUCCH resources corresponding to EPDCCH detections can then be sequentially numbered after the ones corresponding to PDCCH detections.    c) A shared set of PUCCH resources is used and the NodeB scheduler is restricted in using the first CCE for a PDCCH transmission or the first ECCE for an EPDCCH transmission so that the respective HARQ-ACK signal transmissions do not use same PUCCH resources.
The first two approaches increase PUCCH overhead compared to using only PDCCHs for scheduling PDSCHs even though an average number of such PDSCHs per subframe may not be larger than when both PDCCHs and EPDCCHs are used. The first approach results in a larger increase in PUCCH overhead as, if a UE does not read the PCFICH, it may need to assume the largest number of CCEs for PDCCH transmissions. The third approach may avoid increasing the PUCCH overhead but may place significant restrictions on the scheduler operation, which may not be feasible in practice.
Therefore, there is a need to define PUCCH resources for HARQ-ACK signal transmissions in response to detections of PDCCHs, distributed EPDCCHs, and localized EPDCCHs associated with respective PDSCHs, while minimizing the associated overhead and avoiding using the same PUCCH resource for multiple HARQ-ACK signal transmissions.
There is also a need to allocate different PUCCH resources for HARQ-ACK signal transmissions from different UEs in response to respective EPDCCH detections associated with respective PDSCHs and sharing a same first ECCE.
There is a further need to enable antenna diversity for the transmission of a HARQ-ACK signal in response to an EPDCCH detection associated with a PDSCH.